Rrrrachel, I understand what you are saying, titers are really quite meaningless, you can be immune to a disease without evidence of antibodies because that disease was contracted naturally and the body developed cell mediated immunity. Vaccination can only provide a humoral (antibody mediated) immune response which we all know wanes and in some cases pretty quickly (mumps, pertussis, flu etc).
I have never had chicken pox even though I have been exposed countless times as a child and an adult. I might well no have varicella antibodies (no idea never tested and never will), but I certainly appear to be immune to chicken pox.
Frankly, I would be totally against schools requiring titer testing as proof of immunity, because it is not definitive proof by any means.
As would I, for the same reason. Also, non-immune doesn't mean immediate risk...active infection with a contageous disease is what is risky to the public.